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Impulse engines on star ships, based on placement, appear to be reaction engines that propel the vessel by projecting energy on a course opposite of the desired direction of motion.
This is backed up by Memory-Alpha which states:

In Federation starships, the impulse drive was essentially an augmented fusion rocket ... a vectored thrust nozzle to direct the plasma exhaust.

If this is the case then, why do all the star ships only have exhaust ports on the "rear" of the ship? In order to slow down or stop the ship would need to apply a change in velocity opposite to the current direction of motion. Without forward facing exhaust nozzles the ship would need to turn around to slow down, but this is never seen in any of the shows or movies (that I can recall anyway).

The ships do have maneuvering thrusters, but they always seem to be used for low speed maneuvering, and it is not implied that they have the same output as the impulse engines. Using them to stop a star ship moving at full impulse would probably take a very long time. Additionally, there is not any heat scoring on the ship hulls around the impulse engines, which might have suggested exhaust deflectors for braking action.

It would seem, therefore, that there is another technology in place to slow down a star ship at impulse speed, but as far as I can remember such a thing is never mentioned.

Is this ever mentioned in any canon source, regarding how a ship at impulse slows down?

Xantec
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  • I believe there is another question on this SE asking what the glowing red lights on the front of Starships are, with the explanation being that they are reverse-impulse engines. I can't find it at the moment but that might be your answer. – Zibbobz Mar 20 '14 at 18:52
  • If dinky lights worked to slow the ship down, then there would only need to be dinky lights in back as well. – Oldcat Mar 20 '14 at 19:01
  • Doesn't Pike refer to the "External Inertial Dampener" in the new film. Perhaps this can be used to brake the ship in space – Valorum Mar 20 '14 at 20:46
  • @Richard Which new film? The two most recent ones did not feature any of the TNG crew. – Xantec Mar 20 '14 at 20:59
  • @Xantec - The famous (if slightly cringeworthy) Parking brake scene; http://youtu.be/WLHO_E_U8o4?t=51s – Valorum Mar 20 '14 at 21:03
  • Oh, I read Pike as Riker. My mistake. – Xantec Mar 20 '14 at 22:13
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    Q did it – Izkata Mar 20 '14 at 23:15
  • Even if we never see the process of them turning around to decelerate at impulse, if you just see a shot of the ship from the front, moving at impulse so the stars aren't streaking by, there's really no way to tell if it's decelerating (i.e. accelerating in a direction opposite to its velocity relative to the galaxy) or accelerating, it's not like you can't put the imaginary camera at the front of the ship when it's decelerating. So I don't think anything has been seen on the show that's inconsistent with the obvious solution that they just turn the ship around to decelerate. – Hypnosifl Mar 18 '16 at 01:21

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Canonically, impulse engines are reaction engines, but this can’t possibly be correct, even by Star Trek’s loose standards of scientific plausibility. If they were reaction engines, every starship would be mostly fuel tank, like real rockets are.

To illustrate the problem, let’s calculate the reaction mass requirement for one in-system maneuver, accelerating from low Earth orbital velocity (~ 7km/s) to “half impulse” (~ 0.25‌c, per this answer). The mass of the Enterprise-D is said to be 4,500,000 metric tons; let’s assume that that doesn’t include propellant. Since the ship is powered by matter-antimatter annihilation, impulse exhaust could consist entirely of photons, which would make c itself the exhaust velocity. (If the exhaust velocity is slower, the mass requirement goes up.) The final velocity is low enough that we can use the non-relativistic rocket equation. It gives a propellant mass requirement of 1,278,000 metric tons, exactly half of which must be antimatter. That is ~28% of the base mass of the ship. To speed up and then slow down again you need more than double that much mass, because you have to bring along the propellant to be used to slow down. The cost keeps going up for every additional maneuver in between refuelings.

It should be clear that none of the ships shown on Star Trek are carrying around anything like the amount of tankage they would need. Impulse engines must somehow not be subject to Newton’s laws of motion, and at that point, there is no reason to think they would need exhaust nozzles aimed opposite the direction you want the ship to go.

For more on this topic, start with the essay “The Tyranny of the Rocket Equation” and the book Ignition! (PDF of scanned full text). I don’t mean to pick on Star Trek specifically; ignoring or handwaving the propellant requirements of reaction engines is nigh-ubiquitous in space SF. I will, however, observe that Isaac Asimov’s The Gods Themselves tackles it head-on in an interesting way.

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zwol
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    Matter replicator fuel "tanks"? :) – Brian S Mar 20 '14 at 20:18
  • @BrianS Would not help. The tyranny of the rocket equation comes directly from having to accelerate the fuel you'll use later. Perhaps you could store the energy required to replicate the fuel in less overall volume than the fuel would have taken, but its mass is exactly the same, because that's what mass-energy equivalence means. – zwol Mar 20 '14 at 20:51
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    Not in a soft-SF series like Star Trek it doesn't. ;) – Brian S Mar 20 '14 at 21:10
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    @BrianS The question only makes sense if you're insisting on some degree of realism. – zwol Mar 20 '14 at 22:06
  • Enterprise technical drawings show Bussard collectors, which could be handwaved into a Trek solution to the fuel problem. – Kyle Jones Mar 20 '14 at 23:58
  • In addition to providing thrust, the impulse engines also emit a subspace field that reduces the mass of the ship. – Brian Ortiz Mar 21 '14 at 05:27
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    Damnit Jim! Don't bring your sciency facts in here. This is Star Trek! Science has no place here! :) – BBlake Mar 21 '14 at 14:08
  • @zwol The rocket equation has little meaning in space, where there is no gravity to fight against, and mass therefore has little effect. – Tom Lint Mar 14 '16 at 19:45
  • @TomLint Wouldn't that be nice? Unfortunately, the tyranny of the rocket equation is mostly about inertia, not gravity. The more reaction mass you're lugging around, the more of it you have to expend to change your velocity vector. And if you're near a star and its planets, you still have gravity to worry about, too. – zwol Mar 14 '16 at 19:57
  • @TomLint ...Having refreshed my memory of the essay I linked to, it does make it sound like gravity is the dominant factor driving up costs, but that's because the author is thinking only in terms of getting mass from Earth's surface into orbit. For the sorts of things Enterprise does casually with the impulse engines, inertia would be the dominant factor. And if impulse engines were reaction engines, inertia alone would still provide an exponential term and the ship would still wind up being mostly fuel tanks. – zwol Mar 14 '16 at 20:03
  • @TomLint For concreteness: Wikipedia says the mass of the Enterprise-D is 4,500,000 metric tons, and this answer says 0.25c is in the normal range of speed for impulse drive. Let's assume 0.25c is also the exhaust velocity of an impulse engine. To accelerate to 0.25c from low Earth orbital velocity (7 km/s), the rocket equation says 7,730,000 metric tons of propellant expended. That's one maneuver. – zwol Mar 14 '16 at 20:23
  • @TomLint If the exhaust velocity is identical to ''c'' (as is in fact plausible for a drive powered by antimatter—all the exhaust is photons) that gets it down to 1,278,000 metric tons, which is certainly an improvement, but still you can see that there's nowhere to keep that on the ship as shown, and they would constantly be having to refuel. Wolfram Alpha's rocket-equation calculator appears to be non-relativistic but the correction is pretty small below 0.5c. – zwol Mar 14 '16 at 20:30
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    @zwol According to a book I have on the Enterprise, this problem is handwaved away by claiming that the impulse engines use subspace fields to drastically increase the mass and velocity of the propellant as it exits the engine. The obvious conservation of energy violation here notwithstanding, this is how it gets so much thrust from so little fuel. – Timpanus Mar 15 '16 at 01:45
  • @Timpanus - The TNG Technical Manual says on p. 75 that they supplement the fusion engine with a "compact space-time driver coil, similar to those standard in warp engine nacelles, that would perform a low-level continuum distortion without driving the vehicle across the warp threshold." And p. 77 elaborates that the effects of the coil are that it "(1) reduces the apparent mass of the spacecraft at its inner surface, and (2) facilitates the slippage of the continuum past the spacecraft at its outer surface." – Hypnosifl Mar 17 '16 at 16:01
  • Also, p. 21 of the U.S.S. Enterprise Owner's Workshop Manual says "The fusion reaction generated superheated plasma, which was then channeled through subspace compression coils, which increased the apparent mass of the plasma exhaust products, so the resulting forward propulsion of the vessel was as if a much greater mass of plasma had been expelled. This greatly reduced the fuel (and mass) requirements for impulse propulsion." – Hypnosifl Mar 17 '16 at 16:05
  • @Hypnosifl So, this is kind of flailing toward a repudiation of conservation of momentum, while trying to preserve the reaction-ness of the engine -- but I find this actually less plausible than if they'd said "the same warp-field technology used to permit FTL can be applied at sublight velocities to negate conservation of momentum, which means we don't need reaction mass at all anymore." – zwol Mar 17 '16 at 16:11
  • @zwol - I agree, it would have made more sense in terms of Trekphysics if they just dropped the idea that sublight travel involves shooting out exhaust...my guess is that when they wrote "impulse engines" into the original series, the writers hadn't thought about the impossible fuel requirements (or didn't care), then when later writers (like Okuda and Sternbach who did the TNG Tech Manual) wanted to get a little more careful about respecting real physics as much as possible, "impulse engines" were too much of a baked-in tradition to Trek starships to drop. – Hypnosifl Mar 17 '16 at 16:20
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It is performed in the same manner that jet aircraft today slow down after they hit the runway. They deflect the thrust of the engines so that it is expelled in a generally forward direction rather than backwards. Just as jets today don't require the engine to turn around, it isn't required in starships either.

I remember it begin detailed in one or two of the books and/or technical manuals, but I don't recall which. It's been years since I read it.

BBlake
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Braking Thrusters

From Memory Alpha, thrusters in general:

Thrusters were propulsion devices used aboard starships that generated very little speed, but were useful in various situations. It was possible to use maneuvering thrusters to alter a ship's course, to rotate about an axis, or to move short, precise distances without the use of impulse drive. It was possible to use acceleration thrusters to increase the speed of a ship. Starship personnel could manually use thrusters through thruster control.

And braking thrusters specifically:

The braking thruster was a type of thruster. These thrusters were used to brake a starship's speed. If the crew was incapacitated, say, by time travel, these thrusters could fire automatically.

In 1986, after traveling successfully to this year, the HMS Bounty's braking thrusters fired as the starship approached Earth. Later, in 2286, after another successful time travel, these thrusters fired again as the ship approached Earth.

Note that this is less of an issue than one probably thinks, considering that the ships are usually moving via non-Newtonian mechanics (Warp) and rely on impulse only for combat maneuvering and docking.

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joshbirk
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    Considering the impulse drives can get to at least 2/3 the speed of light, or possibly as high as 80%, those thrusters would need to be massive. – Xantec Mar 20 '14 at 20:29
  • Yeah, but that is because impulse is anything non-Warp, i.e sublight. Warp-enabled ships aren't depicting whizzing around at 80% of sublight and then coming to a full stop quickly, or I can't remember an instance. – joshbirk Mar 20 '14 at 20:34
  • @joshbirk Impulse is a specific type of locomotion for starships. It just so happens that they cannot travel at sub-light speed with warp drives (to the best of my knowledge), and the impulse engines are not capable of bringing it to the speed of light. – forest Mar 14 '21 at 04:24
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Impulse engines don't provide enough thrust as fusion rockets, they contain warp coils to lower the ship's inertial mass so it can be moved, think of the impulse engine as a miniature sub-light warp drive. They are fusion reactors that power warp coils, rather than rockets. Though they have vectored exhaust, it only accounts for some of the thrust.

According to the TNG Technical Manual, a good portion of the upper half of the engineering section and interconnect was fuel tank. The fuel is stored in a semi-solid ("slush") state taking up far less space than would traditionally be required with liquid propellants.

See TNG Technical Manual pages 77-80, and 69.

The exhaust vanes would be able to turn 90 degrees to provide a surface to push against, similar to the way jet engines reverse their thrust (clamshell reverser). But the slowing down would mostly be achieved the same way a ship drops out of warp, by a sudden raising of its inertial mass.

Moogle
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anoxm
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In star trek their technology permits them to augment subspace to serve a variety of Uses. The TNG technical manual states the Ships primary impulse engines have a "Vector thrust Director" Like real world thrust vectoring that redirects exhaust in a direction permitting turning. As for reversal.... One argument is that impulse is merely a form of "Sub-warp" of manipulated space time but no where near the grand scale of a warp reaction. Since the ships also have the technology to overcome the inertia of fast acceleration (inertial dampeners) without it's people being smashed into jelly against the walls; it suggests that the same tech is used to redirect it to make thrust in the impulse engines go different direction.

An "Electromagnetic rudder". Starships also have artificial gravity. Since up/down is a matter of perspective; manipulating gravity is within their purview they may be able to augment gravity in any direction they please producing "Attraction" among mass.

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They rotate the ship and point the rear in the direction of travel.

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    That would make the most sense, but I can't remember ever seeing them do that (which I mentioned). If you can find an example I'll accept it, but otherwise I have to believe that they don't do that. – Xantec Mar 20 '14 at 18:58
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    They don't show Picard use the toilet either, but I imagine that this still happens nonetheless. – Oldcat Mar 20 '14 at 19:02